The invention relates to a method of prestressing a component or material and particularly to a method of prestressing an aerofoil, such as an aerofoil section of a gas turbine engine compressor or turbine blade or vane. The invention further relates to a prestressed component and particularly to a prestressed aerofoil, such as an aerofoil section of a gas turbine engine compressor or turbine blade or vane.
Gas turbine engine components are susceptible to damage caused by foreign object ingestion and general fatigue. Such damage may result in stress concentrations and cracks which limit the components"" lives. This is a particular problem in aerofoil leading and trailing edges in both compressor and turbine blades and vanes. One known solution is to increase the thickness of the aerofoil section in the leading and trailing edges. However, this adds weight and adversely affects the aerodynamic performance of the blade, reducing the efficiency of the engine.
It has also previously been proposed to introduce regions of residual compressive stress into aerofoils, ideally resulting in the entire cross-section of the leading and trailing edges being under compression. By creating such xe2x80x9cthrough thickness compressionxe2x80x9d whereby the residual stresses in the edges of the aerofoil are purely compressive, the tendency for cracks to grow is severely reduced. The stress field is equalised out in the less critical remainder of the blade.
Prior U.S. Pat. Nos. 5,591,009 and 5,531,570 disclose a fan blade with regions of deep compressive residual stresses imparted by laser shock peening at the leading and trailing edges of the fan blade. The method for producing this fan blade includes the use of multiple radiation pulses from high power pulsed lasers producing shock waves on the surface of a work piece. However the processes disclosed in these prior patents have a number of disadvantages. The magnitude and the penetration depth of the induced stresses is limited, while the process is generally time consuming, costly and restricted to areas which have optical access. Laser shock peening can typically provide a penetration depth of 1 mm.
According to the invention there is provided a method of prestressing a material, the method including the step of using an electrical discharge or current to produce a pressure pulse in the material or in a medium adjacent the material, the pressure pulse impacting a surface of the material to produce a region of compressive residual stress within the material.
Preferably the electrical discharge or current generates a plasma in the medium. The medium preferably comprises a liquid such as oil or water.
The electrical discharge or current preferably has an energy of at least 35 J and a duration of less than 40 ns.
Preferably the pressure pulse produces an impact pressure of at least 15 GPa on the surface of the material.
The electrical discharge or current may be provided between electrodes. The electrodes may be located between the material and a fixed means for containing or reflecting the pressure pulse.
The electrodes may be located remotely from the surface of the material and the method may include the step of directing the pressure pulse towards the surface of the material. The method may include the step of providing focusing means in the form of a reflector.
The method may include the step of concentrating the pressure pulse as it approaches the surface of the material. The method may include the step of providing concentrating means of a material through which the pressure pulse travels faster than it does in the medium, a sectional area of the concentrating means remote from the surface of the material being greater than a sectional area of the concentrating means adjacent the material.
The pressure pulse may be produced by direct impact of the electrical discharge or current on the surface of the material. The method may include the further step of removing a damaged, sacrificial layer from the surface of the material.
The method may include the step of providing a conducting membrane over a surface of the material and providing the electrical discharge or the current through the conducting membrane.
The material may comprise part of an aerofoil section, which may form part of a compressor or turbine blade or vane. Preferably the pressure pulse impacts at least one of a leading and a trailing edge of the aerofoil section. Preferably the method includes the steps of producing a pressure pulse which impacts a suction side of the leading or trailing edge and producing a pressure pulse which impacts a pressure side of the leading or trailing edge, the respective pressure pulses impacting substantially simultaneously.
The material may include an orifice, the inside surfaces of which are to be prestressed and the method may include the step of providing electrodes within the orifice. The method may further include the step of providing a tube of a non-conductive material within the orifice, the electrodes being contained within the tube.
According to the invention there is further provided apparatus for prestressing a material, the apparatus including a medium within which or adjacent to which the material may be located and means for providing an electrical discharge or current to produce a pressure pulse in the medium for impacting a surface of the material to provide a region of residual compressive stress within the material.
The composition of the medium may be such that a plasma may be generated by the electrical discharge or current. The medium preferably comprises a liquid such as water or oil.
Preferably the means for providing an electrical discharge or current is capable of providing a discharge or current having an energy of at least 35 J and a duration of less than 40 ns.
The means for providing an electrical discharge or current may include a pair of electrodes located at least 1 mm from the surface of the material. The electrodes may be located between the material and a fixed means for containing the pressure pulse.
The electrodes may be located remotely from the surface of the material and the apparatus may include means for directing the pressure pulse towards the surface of the material. The apparatus may include focusing means in the form of a reflector, the electrodes being located generally between the reflector and the surface of the material.
The apparatus may include concentrating means of a material through which the pressure pulse travels faster than it does in the medium, a sectional area of the concentrating means remote from the surface of the material being greater than a sectional area of the concentrating means adjacent the material.
The apparatus may include a conducting membrane for covering a surface of the material, and receiving the electrical discharge or current.
According to the invention there is further provided a material including a region of compressive residual stress produced by a method according to any of paragraphs five to fifteen above. The region of compressive stress may be provided in an area which is particularly subject to fatigue damage, foreign object damage, cavitation damage or erosion damage. The material may have been repaired prior to the production of the region of compressive stress.
The material may comprise part of an aerofoil section of a compressor or turbine blade or vane for a gas turbine engine. Preferably the region of compressive residual stress is provided within at least one of the leading and trailing edges of the aerofoil section. A region of residual compressive stress may be provided on both of a suction and pressure side of the leading or trailing edge of the aerofoil section.
The region of compressive residual stress may extend at least 1 mm into the material.